This invention relates to ZSM-5 zeolites having uniformly large crystals, the synthesis thereof and the use thereof as catalysts in organic compound conversions.
Zeolitic materials, both natural and synthetic, have been known in the past to have catalytic capability for various types of hydrocarbon conversion reactions. Certain of these zeolitic materials comprising ordered porous crystalline aluminosilicates have a definite crystalline structure, as determined by X-ray diffraction, within which there are a number of small cavities which are interconnected by a number of still smaller channels. These cavities and channels are precisely uniform in size within a specific zeolitic material. Since the dimensions of these pores are such as to accept for adsorption purposes molecules of certain dimensions while rejecting those of larger dimension, these materials have commonly been known to be "molecular sieves" and are utilized in a variety of ways to take advantage of the adsorptive properties of these compositions.
Crystalline aluminosilicates have been characterized by the presence of aluminum and silicon, the total of such atoms to oxygen being 1:2. The amount of alumina present in conventional aluminosilicates appears directly related to acidity characteristics of the resulting product. Low alumina content is advantageous in attaining low acidity, desirable for low coking and low aging rates.
U.S. Pat. No. 3,941,871 discloses the preparation of zeolites such as ZSM-5 having a high SiO.sub.2 to Al.sub.2 O.sub.3 ratio. The disclosure of this patent includes a teaching of the combined use of tetrapropylammonium (TPA) ions and tetramethylammonium (TMA) ions to make a crystalline metal organosilicate. U.S. Pat. No. 3,849,463 discloses a method of decreasing silica occlusion in a zeolite having a SiO.sub.2 /Al.sub.2 O.sub.3 ratio of greater than 6 by incorporating in the reaction mixture an alkali metal salt.
The Dwyer et al U.S. Pat. No. 4,375,458, the entire disclosure of which is expressly incorporated herein by reference, describes a method for making large crystallite zeolites. More particularly, this Dwyer et al patent describes a method for preparing a crystalline zeolite having a SiO.sub.2 to Al.sub.2 O.sub.3 molar ratio of from about 25 to 1000 and having a crystal size at least 1 micron, the method comprising having in the reaction mixture (1) a combination comprising two different alkylammonium cations or (2) a combination comprising an alkylammonium cation and a metallic cation, wherein in combination (1) at least one of the alkylammonium cations and in combination (2) at least the metallic cations have an ionic radius of from 1.40 A to the pore size of the specific zeolite. Preferably this will be a combination of TPA and a member selected from the group consisting of TMA, tetraethylammonium (TEA), cesium and rubidium cations. The Dwyer et al U.S. Pat. No. 4,375,458 also indicates that the mixture may also have therein added alkali or alkaline earth metals.